Coal represents our largest resource of fossil energy. The efficiency of converting this stored chemical energy to commonly used electrical energy can be significantly improved if the coal is first gasified and the resulting hot coal gas further oxidized in either a fuel cell or in a heat engine. This approach is presently being pursued vigorously by the United States Department of Energy.
One of the major problems encountered in this approach is the presence of sulfur in most of the coals, which gets converted to hydrogen sulfide during gasification. The concentration of this extremely toxic hydrogen sulfide gas in the hot coal gas at the several thousand ppm or higher level is unacceptable from the environmental point of view. Such high concentrations of hydrogen sulfide gas is also undesirable from the process economics point of view because the gas is corrosive to equipment and instruments and adversely affects the performance of molten carbonate fuel cell. It is projected that for such applications, the hydrogen sulfide concentration level should be on the order of a few ppm or less.
A similar situation is encountered in the steel industry where coal is converted to coke, which is then used in the making of iron. Hydrogen sulfide formed during the coking of sulfur-bearing coal has to be removed from the hot coke oven or producer gas before it can be used further.
The conventional technology of scrubbing the gas for removal of hydrogen sulfide is not practical in these situations because the scrubbing processes operate at room temperature or relatively low temperature and, therefore, impose a severe thermal penalty. Thus for the integrated gasification combined cycle approach, the desulfurization has to be performed at high temperature and, in many cases, at high pressure. To improve the process economics further, it would be desirable to have an easily regenerable sorbent which would not only decrease the cost of sorbent but also the costs associated with frequent loading and unloading of the reactors with sorbent and the costs associated with disposal of the used sorbent. Regeneration, of course, means putting the sorbent back into the oxide form, and this entails oxidation of sulfur present as sulfide. It would be desirable if this sulfur is recovered in a commercially marketable form such as elemental sulfur, liquid sulfur dioxide, or sulfuric acid. This would be possible if a concentrated sulfur dioxide stream is generated during the regeneration cycle.
The high temperature desulfurization can be conveniently accomplished by using solid sorbents such as oxides of those metals that form stable sulfides. Calcium oxide in the form of calcined or half-calcined limestone or dolomite can be an obvious choice. However, in this case it is very difficult to remove the sulfur from the sorbent and convert it back to the oxide form for repeated use because of the stability of calcium sulfate formed during oxidation of the calcium sulfide. Another problem is encountered if one decides to throw away the calcium sulfide rather than try to regenerate it at considerable expense. This problem is related to generation of hydrogen sulfide gas when calcium sulfide is exposed to humid environment.
To overcome this problem, the Morgantown Energy Research Center of the U.S. Department of Interior, now known as Morgantown Energy Technology Center of the U.S. Department of Energy (METC), developed some iron oxide based sorbents for high temperature desulfurization, as detailed in U.S. Pat. No. 3,579,293 granted to Shultz, et al. Since iron sulfate decomposes to iron oxide at moderately high temperatures, the sorbent is regenerable. The iron oxide was mixed with fly-ash and bentonite to impart strength to the extrudates.
During mid 1970's, the Morgantown center extended the applicability of iron oxide based sorbents to removal of hydrogen sulfide from hot, low-Btu gas. The hydrogen sulfide concentration in the feed gas was about 0.5 percent, and about 90 to 94 percent removal was accomplished, resulting in about 300 to 500 ppm hydrogen sulfide in the exit gas stream. The absorption capacity of the sorbent was about 8 percent sulfur by weight.
The removal of hydrogen sulfide to the 300 to 500 ppm level is not adequate for most of the energy conversion options currently under investigation by the Department of Energy and other companies interested in producing and utilizing hot coal gas. In fact, the tolerance for a molten carbonate fuel cell (MCFC) may be only a few ppm, and to meet this requirement, METC tested several sorbents containing zinc oxide by itself or in combination with iron oxide. It was concluded that either zinc oxide or zinc ferrite can be used to desulfurize hot coal gas to a few ppm H.sub.2 S level. Such low hydrogen sulfide concentration levels are not attained with other metal oxides such as manganese oxide recommended in U.S. Pat. No. 4,180,549, granted to Olsson, et al.
The choice of zinc oxide was based on the thermodynamic considerations that indicated very low concentration levels of H.sub.2 S in equilibrium with ZnO, ZnS, and H.sub.2 O vapor. However, during the regeneration cycle, zinc sulfate is somewhat more stable than iron sulfate at the same temperature. Thus, the zinc ferrite sorbent, which represents a stoichiometric combination of Fe.sub.2 O.sub.3 and ZnO, offers the best properties of both oxides and was selected for further development work at METC.
During further testing of zinc oxide and zinc ferrite sorbents at METC, it was observed that the sulfidation capacity of the sorbents dropped significantly during the second, third, and subsequent cycles of testing.
Such quick deterioration in the performance of the sorbent cannot be accepted in commercial applications where the plant has to perform on design capacity cycle after cycle, day after day for long periods. It is estimated that this period should be about one year for an economically attractive operation. Besides the loss in sulfur sorption capacity, the presently available sorbent also undergoes physical disintegration. Such disintegration would not only cause deterioration in the performance of the reactor but also would lead to loss of sorbent fines from the bed, which in turn would require another device to separate the fines from the gas before it could be used in heat engines or fuel cells. Another shortcoming of some of the state-of-the-art sorbents is that they are prepared with a large amount of inert material which occupies space in the reactor but does not contribute to the desulfurization process. This inert material has been used or recommended either in the form of support material, as described in U.S. Pat. No. 4,089,809 granted to Farrior, Jr., or in the form of inert filler, as described in U.S. Pat. No. 4,088,736 granted to Courty, et al.
The preparation of sorbents using complex procedures of supporting them on filler base, or forming extrudates, or subjecting the extrudates to a variety of impregnation and thermal treatments makes the sorbent production process expensive. The cost would be further increased when a number of reagents are required or the reagents are not commercially available common chemicals.
Other known references to the subject matter of the invention include the following:
1. E. C. Oldaker, A. M. Poston, Jr., and W. L. Farrior, Jr., "Removal of Hydrogen Sulfide from Hot Low-Btu Gas with Iron Oxide-Fly Ash Sorbents", Report MERC/TPR-75/1, Morgantown Energy Research Center, Morgantown, W. Va., February 1975. PA0 2. T. Grindley and G. Steinfeld, "Development and Testing of Regenerable Hot Coal Gas Desulfurization Sorbents", Report DOE/MC/16545-1125(DE82011114), Morgantown Energy Technology Center, Morgantown, W. Va., October 1981. PA0 3. T. Grindley and G. Steinfeld, "Zinc Ferrite Hydrogen Sulfide Absorbent" in Third Annual Contaminant Control in Hot Coal Derived Gas Streams Contractors' Meeting Proceedings, K. E. Markel (Editor), DOE/METC/84-6(DE84000216), December 1983, pp. 145-171. PA0 4. T. Grindley and G. Steinfeld, "Testing of Zinc Ferrite Hydrogen Sulfide Absorbent in a Coal Gasifier Sidestream" in Proceedings of the Fourth Annual Contractors' Meeting on Contaminant Control in Hot Coal-Derived Gas Streams, K. E. Markel (Editor), DOE/METC-85/3(DE85001954), December 1984, pp. 314-336. PA0 5. P. R. Westmoreland, J. B. Gibson, and D. P. Harrison, "Comparative Kinetics of High-Temperature Reaction Between H.sub.2 S and Selected Metal Oxides" Environmental Science and Technology, Volume 11, No. 5, May 1977, pp. 488-491. PA0 6. R. A. Swalin, Thermodynamics of Solids, John Wiley and Sons, Inc., New York, pp. 306-312. PA0 7. Y. K. Rao, "Catalysts in Extractive Metallurgy", Journal of Metals, July 1983, pp. 46-50.